Tissue-specific promoters and enhancers play a major role in the control of developmentally regulated gene expression during development. Expression of rearranged immunoglobulin (Ig) genes introduced into both lymphoid and non-lymphoid cells has led to the identification of tissue-specific transcriptional enhancer sequences in the major intron between the J and C region of the Ig gene. We have shown that the Ig promoter also contributes to tissue-specific expression of mouse Ig kappa gene. Tissue-specificity of Ig gene enhancers and promoters is thought to be due to their interaction with trans-acting regulatory factors. We have designed a genetic approach to study and clone such B cell specific trans-acting factors. Plasmid vectors we've constructed that contain the bacterial neomycin resistance gene linked to the mouse immunoglobulin kappa gene promoter and a neutral enhancer or Ig heavy chain gene enhancer. By introducing these hybrid genes into non-lymphoid mouse 3T3 and L cells, we have created recipient cell clones in which the hybrid gene is stably integrated and non-functional. The hybrid gene could then be activated by cell fusion and by DNA transfer methods. Our results suggest that this approach can be used to directly isolate genes that code for putative trans-acting regulatory factors. Isolation of these regulatory genes will greatly enhance our ability to understand the regulation of tissue-specific genes at the molecular level.